Injection of liquid fuels into shaft furnaces

ABSTRACT

Method and apparatus capable of atomizing and mixing large, even stoichiometric, amounts of liquid fuel (with or without oxygen) in the blast of tuyeres supplying a shaft furnace without ignition of the mixture prior to reaching the furnace, accomplished by atomizing the fuel by injection into a blast having a speed of between mach 0.3 and mach 1 and thereafter reducing the speed of the blast to velocities acceptable for supplying the furnace requirements, before ignition of the fuel can occur, by constantly increasing the cross-section of the tuyere conduit in a smooth curve to give a flow path without recirculation streams. In a further aspect of the invention, any generatrix of the conduit is wholly convex with the tangent at the discharge end forming an angle of at most 15* to the axis of generatrix.

Bruhlet et al.

[451 May 7, 1 974 INJECTION OF LIQUID FUELS INTO SHAFT FURNACES [75]Inventors: Paul Bruhlet, Nilvance; Gerard Quillet, Thionville, both ofFrance [73] Assignee: Compagnie Francaise de Raffinage,

Paris, France 22 Filed: July 7, 1972 211 Appl. No.: 269,556

[30] Foreign Application Priority Data- FOREIGN PATENTS OR APPLICATIONS537,892 5/1922 France 239/433 154,674 5/l95l Australia ..239/433 PrimaryExaminer-Carroll B. Dority, Jr. Attorney, gent, or F irm-Curtis, Morris& Safford; A. Thomas S. Saffqrd 57 ABSTRACT Method and apparatus capableof atomizing and mixing large, even stoichiometric, amounts of liquidfuel July 8, 1971 France 71.25093 (with or without oxygen) in the blastof tuyeres supplying a shaft furnace without ignition of the mixture[52] US. Cl 431/181, 75/42, 266/30, prior to reaching the furnace,accomplished by atom- 266/ 9/433 izing the fuel by injection into ablast having a speed [51] Int. Cl. F23c 5/08 of between machO 3 and machl and thereafter rel Field-0f Search ducing the speed of the blast tovelocities acceptable 239/4 4.5; 431/ 181, 75/42 for supplying thefurnace requirements, before ignition of the fuel can occur, byconstantly increasing the References Cited cross-section of the tuyereconduit in a smooth curve UNITED STATES PATENTS to give a flow pathwithout recirculation streams. In a 290,343 12/1883 Morgan et al. 75/42furthe? aspect 0f the i any generatrix W 966,704 8/1910 'Pickles 239 433condult 1S Wholly Convex with the Hinge"t a! the 3,558,! 19 'l /1971Demalander 75/42 a ge end form ng an angle of at most 15 to the axis3,596,894' 8/1971 Duthion et al 75/42 of generatrix. 3,608,881 9/197!Yordanov et al. 266/41 18 Claims, 3 Drawing Figures 6 w 77% D I, iiii z?1 of D i A 4 X'- A X l INJECTION OF LIQUID FUELS INTO SI-IAFI FURNACESThe present invention relates to the injecting of liquid fuels into ashaft furnace, and more particularly to the injection of fuel into ablast furnace.

Methods and devices are known for injecting fuel into the nozzle ortuyere of a shaft furnace in which the fuel is injected into the blastvia injectors or blowpipes under a relatively high pressure. The averagesize of the injected droplets of fuel is generally too large and thecombustion becomes defective when the rate of the injection of the fuelis increased.

It has been attempted to increase the fuel injection rate up tostoichiometric conditions and even beyond, while retaining goodcombustion, by atomizing the fuel in the form of fine droplets andpartially or totally burning the atomized fuel, mixed with the blast, inthe nozzle or the tuyere.

A process is known in which the combustion also takes place in thenozzle or tuyere, the holding of the I flame being assured in thisprocess by a physical obstacle or by a sudden widening which creates arecirculation zone which captures and stabilizes the flame (FrenchPatent 1,558,425 or its two patents of addition corresponding to US.Pat. No. 3,596,894).

Another process is known in which the fuel is burned in the nozzle ortuyere, which forms a combustion chamber, where the holding of the flameis assured by a prior rotating of the blast (French Patent 1,559,679).

In addition, a process is known in which the fuel is injected into atuyere, preferably in the formof a Laval tuyere. The fuel, previouslyatomized by an'auxiliary gas, is injected therein into the blast, in aconvergence. The convergence isfollowed by a divergence of linearprofile, the'half angle of which is between and 7.5, and preferablybetween 0 and 35. This process has the purpose of imparting to theblast-fuel mixture at speed of departure from the tuyere which is equalto or greater than mach 1 so as to permit the penetration of saidmixture to the center of the blast furnace (Belgian Patent 39,431).

Furthermore,-other processes are known in which the fuel is burned in areactor outside the shaft furnace, the burned gases in thiscase beinginjected through the tuyeres into the hearth.

Some of these last-mentioned processes make it possible to employ fuelinjection rates which reach or exceed stoichiometric conditions withgood conditions of combustion. On the other hand, they have the drawbackof causing considerable variations in the impulse" of the gasesentering-the hearth, that is to say, the amount of movement, as afunction of the variations in the amount of fuel injected. Thesevariations in impulse cause changes in operation in the shaft furnaces,which changes, if they cannot be corrected, may result in substantialdisturbances when one is compelled to vary the fuel injection ratessubstantially with time.

An object of the present invention is to avoid these substantialvariations in the impulse of the gases entering the hearth as a functionof the amount of fuel injected, which may vary between zero andstoichiometry and even beyond, while assuring the best conditions ofcombustion. .that is to say, reducing the formation of carbon soot to aminimum.

This result is obtained by burning at the nose (i.e., the discharge end)of the tuyeres a "blast-fuel mixture prepared under special conditionsand by avoiding any combustion of this highly inflamable mixture beforeit arrives at the nose of the tuyeres.

For this purpose, an embodiment of the present invention is a processfor introducing through the tuyeres into the hearth of a shaft furnace,a blast mixture, whether with added oxygen or not, and large quantitiesof atomized liquid fuel, in which process:

liquid fuel isatomized by the blast at a speed of be tween mach 0.3 andmach l;

mixture is transported to the nose of the tuyeres within a time which isless than the ignition time of the said mixture;

the speed of the mixture is constantly decreased towards the nose ofthe.tuyeres;-

the said decrease in speed is assured by a conduit a constantlyincreasing cross-section;

recirculation flows of the said mixture are avoided so as to prevent anyignition, said result being obtained by creating a smooth flow paththrough a conduit of constantly increasing crosssection with a curvedprofile, this curved profile being possibly extended by a linearprofile.

Another embodiment of the invention consists of a shaft furnace tuyerefor the carrying out of said process, in which:

the feed conduit for the atomized liquid fuel/blast mixture has theshape of a curved diffuser any generatrix of which is a curve theequation of which has a second derivative which is always positive;

a line tangent to the end of any curved generatrix forms with a linetangent to the point of origin of said generatrix an angle at most equalto 15, and preferably between 10 and 12.

A further embodiment of the invention is a shaft furnace tuyere for thecarrying out of the said process, in

which:

the feed conduit for the atomized liquid fuel-blast mixture has theshape of a curved diffuser any generatrix of which is a curve whoseequation has a second derivative which is always positive, the saidcurved diffuser being extended by a linear diffuser;-

the generatrix of the linear portion forms with a line tangent to theorigin of the curved generatrix with which it connects, an angle at mostequal to 15, and preferably between 10 and 12;

a line tangent to the end of any curved generatrix forms with the linetangent to the origin of said generatrix an angle at most equal to 15and preferably between 10 and 12.

By means of the present invention, the conditions of flow of the fuelinjection blast, and in particular the amount of impulse of the mixtureat the nose of the tuyere and the losses in pressure in the blast feedconduit, are not substantially changed as compared with a blowing of theshaft furnace without fuel injection, whatever the amount of fuelinjected.

The invention furthermore makes it possible to vary the rate ofinjection without modifying the diameter at the nose of the tuyeres. lnparticular, it makes it possible to inject. under good conditions, verylarge quantities of fuel into the shaft furnace and to do this with allthe progressiveness desired.

Other characteristics and advantages of the description will becomeevident from the following detailed description in whichreference is hadto the accompanying diagrammatic drawing. In said drawings, which arenot of a limitative nature:

FIG. 1 is a cross-section through the inner profile of the blast feedconduit at the outlet into the shaft furnace in accordance with theinvention;

FIG. 2 is a section through the same profile, the downstream part ofwhich has'been modified in accordance with adifferent embodiment of theinvention;

FIG. 3 is a longitudinal diagrammatic section through a blast furnacetuyere in accordance with the invention.

The invention consists in feeding to the nose of the tuyere a mixture offuel and combustion-supporting agent, ready to be ignited andburned'intensely upon its arrival at the nose of the tuyeres, withoutprior ignition in the blast conduit and with a minimum cracking of thefuel.

In order to obtain good combustion, it is known that the fuel must beatomized and distributed as uniformly as possible in the blast.

In accordance with the invention, the atomization is effectedeconomically in known manner by injection of the liquid fuel into thehot blast brought to high speed by passage through a portion of conduitof suitable section a.

Moreover, the distribution-of the liquid fuel in the hot blast improvesincreasingly rapidly in the direction of flow, the finer the atomizationis. For this purpose, the speed of the blast in the injection zone mustbe at least equal to mach 0.3, which establishes the upper limit of thecross-section a." i r However, it is also necessary to avoid combustionof the atomized fuel mixed with the blast in front of the nose of' thetuyere so as to prevent the impulse of the jet entering the furnace fromvarying with the amount of fuel injected. For this purpose one couldcontemplate effecting this atomization very close to the nose. However,the velocity of the blast, upon its entrance into the hearth, is afunction of the operating conditions of the shaft furnace of which-itconstitutes one of the adjustment parameters. Now, in general, thespeeds of introduction of the blast used are definitely less than thespeeds which assure good atomization. The outlet cross-section A"-at thenose of the tuyeres, which is imposed by the operating conditions of theshaft furnace, is therefore generally larger. than the cross section apreviously determined.

Inorder to pass from speeds of more than about mach 0.3, which assuresgood atomization, to the lower speeds imposed by the operation of theapparatus, the invention employs a conduit of constantly increasingcross-section which connects the injection zone to the nose of thetuyere.

The mixture of fuel and combustion supporting agent must be transportedto the nose of the tuyere within a time which is at most equal to thetime which would be necessary for the droplets of fuel to ignite byprogressive heating in contact with the hot blast. This maximum time ofstay is equal to the ignition time. In accordance with thecharacteristics ofthe blast and the fuel (temperature, rate of flow,etc.), under the different operating conditions of the shaft furnace,this time of stay leads to limiting the length fL between the injectionzone and the nose of the tuyere. We shall designate by L.- the criticalvalue of this length; LJ is therefore the maximum value of L under givenconditions. I

' In order to avoid the ignition of the atomized fuel mixed with theblast before it reaches the nose of the tuyere, it is also necessary toavoid within the conduit of increasing cross-section anything whichmight cause said ignition by'creating recirculation streams, or anyphonomenon presenting the same drawbacks, and in particular theappearance of a shock wave. In order to avoid this last-mentionedphenomenon, the speed of the blast must at all times remain subsonic.The crosssection a of the blast feed conduit in the injection zone istherefore determined in such a manner thatthe speed of the blast thereis between about mach 0.3 and about mach 1, with due consideration ofthe maximum characteristicsof the blast which are capable of being used(rate of flow, temperature, pressure, etc.).

The conduit of increasing cross-section whose dimensions have just beendetermined must furthermore have an overall shape such as not to createat any point of the flow of the fluid at the wall points of particularloss of pressure where recirculation streams could appear. It isnecessary to reduce the risk of ignition in the boundary layer of thestream of blast flowing in said conduit.

In practice, the limitations imposed by the ratio a/A of the upstreamand downstream cross-sections and the maximum possible length L for thisconduit of increasing cross-section do not make it possible to impart toit the shape of a traditional divergent cone with sufficiently smallangle to avoid any separation. For this reason, the invention proposesimparting to said conduit the shape of a curved diffuser permittinglaminar flow at the wall.

Referring to FIG. 1, the hot blast is fed through the conduit 1 ofcross-section a at a speed of between mach 0.3 and mach The liquid fuelis injected into the zone 2 via one or more injectors which feed thefuel in sheets substantially perpendicular to the flow of the blast. Themixture of blast and atomized fuel is then guided by the curveddiffuser3 up to the nose of the tuyere 4 which debouch es into the furnace 5.

The upstream cross-section a'of the diffuser 3 is the cross-section ofthe blast conduit in-the injection zone 2; the downstream cross-sectionA of the diffuser 3 is that of the nose of the tuyere 4 upon dischargeinto the shaft furnace. The length L is at most equal to the length L asdetermined previously, that is to say, the maximum distance permittedbetween the injection zone and the nose of the tuyere. If the length ofthe curved diffuser is designated by A, then L is equal to X when theinjection is effected at the entrance to the diffuser. L is greater thanA when the injection is effected upstream of the entrance to thediffuser. The generating curves of this diffuser 3 are curvilinear andcharacterized by equations whose second derivative is always positive.Each generatrix has its origin a point B located on the connecting linebetween the point of origin of the diffuser and the blast feed conduitand passes through a point D of the downstream section of the diffuserwhich is imposed by the outlet cross-section of the tuyere.

If the envelope surface of the fuel which has diffused from theinjection zone 2 is inscribed within the diffuser 3, there will be nofuel on the inner wall of the diffuser, which wall will in practice beprotected by a thin layer of blast, avoiding the dangers of ignition inthe boundary layer.

If this envelope surface encounters the wall of the diffuser 3 beforethe end, there will be observed a slight trickling of liquid fuel on theinner wall of the diffuser. However, in order that the danger ofignition remain small in this boundary layer, despite the reduced speedof flow (the velocity gradient varies very rapidly in the vicinity ofthe wall), the negative temperature gradient in this boundary layer isincreased by intense cooling of the wall of the diffuser, and anypossibility of catching of the flame on the wall is eliminated by a verysmooth surface of this wall. I

In either of these hypotheses, the boundary layer must remain very thin.As a matter of fact, it generally creates a heterogeneity ofdistribution of the fuel in the blast, which heterogeneity must bereduced, particularly when the amount of fuel injected is high and closeto stoichiometric.

Furthermore, there is a limit angle of opening of the diffuser 3 beyondwhich there takes place within the boundary layer a turbulent flow whichfavors the igniting of the mixture before it arrives at the nose of thetuyere. Atevery point on a curved generatrix, the angle a defined by thetangent at this point and the tangent at the origin must remain lessthan half of the limit angle. In particular, the angle a as shown inFIG. 1, defined by the lines tangent to the points B and D must be atmost equal to I5", and preferably be between and 12.

If, for the dimensions of the diffuser and the shape of the generatricescontemplated, the half angle of opening .at the point D must be greaterthan this limit, the curvilinear generatrix BD of the diffuser is thenreplaced by a curvilinear generatrix BD extended by a straight line DD,tangent at D to the generatrix BD and forming with the tangent at B toBD an angle equal to said limit angle, as indicated at 6 in FIG. 2. Thepoint D can be determined since it is common to the curvilineargeneratrix BD and to the straight line DD and the tangent at this pointto the generatrix BD is identical with the straight line DD.

The liquid fuel, mixed with the blast, which is guided by the diffuser 3in which it cannot ignite, arrives at the nose of the tuyere where itignites and burns very rapidly under the normal operating conditions ofthe furnace. The variation in impulse of the injected gases as comparedwith a conventional injection does not result in any disturbances in theoperation of the furnace.

In one particular case, the equation of each curvilinear generatrix mayadvantageously be of the form:

(1 the abscissa axis X'X being the axis of flow and the ordinate axisbeing an axis perpendicular to the preceding one in the plane determinedby the generatrix and the axis of the flow.

, A: length of the curved part of the diffuser;-

Y ordinate of point D;

Y,;: ordinate of point B.

In this case, the pressure gradient remains constant in the direction offlow and the losses in pressure are minimal.

The present invention has been reduced to practice on a tuyere havingthe following characteristics:

Diameter at the nose which tuyere was used on a blast furnace having thefollowing characteristics:

Diameter of the hearth 8.80 m Number of tuyeres 20 Average temperatureof the blast Average rate of flow of the blast without additional oxygen6,000 m NTP/hr In FIG. 3, which shows the blast feed device in thehearth of the blast furnace, the tuyere 7 which has the characteristicsenumerated above is cooled in known manner. It has a water chamber 8with one or more water inlets 9 and one or more outlets 10.

The blast conduit of the tuyere 7 having the axis X'X consists fromupstream to downstream of a succession of portions of conduits ofrevolution around the axis X'X. The upstream frustoconical portion is ofconvergent form 13. The area of the inlet cross-section of theconvergent portion 13 is equal to that of the cross section of theconduit of the nozzle 11 and the area of its downstream cross-section isequal to that of the cross-section of the cylindrical portion of adiameter of 1 10 mm forming the neck 12 which follows it. With dueconsideration of the requirements of manufacture, positioning, placingof the injection pipe, cooling, etc., there is sought a convergent shape13 and a length of the neck 12 which result in minimum losses inpressure. The neck 12 has a length of2 I0 mm. The convergence I3 and theneck 12 improve the stabilization of the flow of the blast.

The area of the cross-section of the neck 12 is smaller than that of theconduit of the nozzle II so as to increase the speed of the blast inorder to assure a satisfactory pneumatic atomization of the fuel by theblast. The speed of the blast is then from 350 to 400 m/sec.

h9 ne s. stens qfilown tbya di fuse which is initially curved at 3 overa length k 252 mm and then linear at 6 over a length of 38 mm. Since theinjection in this case is effected at the entrance to the diffuser, thetotal length of the diffuser is equal to the length L. The upstreamcross-section of the diffuser is circular and its diameter is equal tothat of the neck 12. Its downstream cross-section is also circular, witha diameter equal to that of the nose of the tuyere 7.

As the directrices of the curved portion of the diffuser are in thiscase circular, all the generatrices are represented by a singleequation. This equation, by application of formula I, then becomes:

+[(R/ l[ x/A] 2) in which R represents the radius of the downstreamsection of the curved portion of the diffuser and r the radius of theupstream section.

As this diffuser has a linear end part, the radius R is therefore notthe radius of the nose of the tuyere at the point D, but that of theblast conduit at the point D,

and )1 represents the abscissa of this point D.

R 75.5 mm

r 55 mm I The straight line DD forms an angle of 1 1 with the tangent toB' (which in this case is parallel to the axis X'X).

Under conditions of constant production, or a circular blast with atemperature of 950C and a blast containing 25 percent additional oxygen,the production of liquid fuel added being close to stoichiometry, namelyabout 535 1/hr. per tuyere of heavy'fuel oil No. 2, the fuel mixed withthe blast does not ignite within the tuyere but rather directly at thenose of the tuyere in the furnace. The variation of the impulse is verylight and does not disturb the operation of the furnace. The com-.

bustion is much more intense and complete than with the customaryinjection devices. We claim: 1. In a method for blowing shaft furnacesthrough at least one tuyere with a blast capable of containing largequantities of atomized liquid fuel, the improvement for maintaining asubstantiallyconstant impulse at the outlet of the tuyere, whatever theamount of fuel injected, with rapid combustion of the mixture atthe noseof the tuyere, said improvement comprising the steps of:

atomizing the liquid fuel in a stabilized blast in an injection zonewhere the speed of said blast is between mach O.3 and mach l; conductingthe resulting fuel-blast mixture to the nose of the tuyerewithina timewhich is less than v the ignition time of the said mixture;

constantly and smoothly decreasing the, speed of the mixture towards thenose of the tuyere to the desired furnace-blowing velocity by constantlyincreasing the cross-section of the flow path along a curved diffuserprofile the interior surface of which -is always convex and whichprofile at one end connects to the injection zone and at its other endextends towards the nose of the tuyere and the tangent of which profileat said other end connects to said nose, which profile avoidsrecirculation steams. I

2. Method according to claim 1 wherein the tangent to said curvedprofile has finite dimensions such that the said flowpath is extended atthe discharge end by a linear profile.

3. Method according to claim 1 wherein said flow path has an outerboundary whose shape is wholly defined by a generatrix the equation forthe curve of which has a second derivative which is always positive.

4. A method according to claim 2 wherein the curved diffuser profileportion of said flow path has an outer boundary hose shape is defined bya generatrix the equation for the curve of which has a second derivativewhich is always positive.

5. Method according to claim 3 wherein the tangent to the end of anycurved generatrix forms an angle of at most 15 with the tangent to theorigin of said generatrix.

6. Method according to claim 4 wherein the generatrix of the linear flowpath boundary is tangent to the curved generatrix with which itconnectsand forms an angle of at most 15 with the tangent to theoriginof said generatrix.

7. Method according to claim 5 wherein said angle is between 10 and 12.

8. Method according to claim 6 wherein said angle is between 10 and 12.

9. In a shaft furnace tuyere having a discharge conduit of fixed shapewith a narrowed inlet at a fuel injection zone and an enlarged outletnose and means for injecting and atomizing variable amounts of liquidfuel into the blast flowing through said zone and into said dischargeconduit, an improvement in said discharge conduit comprising means forsupplying through said injection zone a substantially stabilized blastat a speed of between mach 0.3 and 1, a curved diffuser with aconstantly and smoothly increasing cross-section throughout its lengthfrom said inlet and extending tangentially at its other end towards saiddischarge outlet nose to flow said blast within said conduit without anyrecirculation andto deliver it at said discharge outlet nose atconventional furnace blowing velocities, said curved diffuser having ashape the generatrix of which is a curve whose equation has a secondderivative which is always positive, and the axial distance from saidinlet to said outlet nose being no greater than L 10. In a tuyereaccording to claim 9, the improvement further comprising a tangentiallinear extension to said diffuser.

11. In a tuyere according to claim 9, the improvement further comprisingthe tangent to the end of any curved generatrix forming an angle of atmost l5 with the tangent to the origin of said generatrix.

12. In a tuyere according to claim 10,- the improvement furthercomprising the generatrix of the linear part being tangent to the curvedgeneratrix with which it connects and forming an angle of at most 15with the tangent to the origin of said curved generatrix.

13. In a tuyere according to claim 11 the improvement further comprisingthe angle being between 10 and 12.

14. In a tuyere according to claim 12, the improve ment furthercomprising the angle being between 10 and 12. g

15. A tuyere according to claim 11, wherein the generatrix of the curveddiffuser is defined by the following equation:

= radius of the discharge end of. the curved difat least 0.3 mach in theinjection zone to wfithin acceptable furnace blowing velocities at saidnose,

r radius of the inlet of the diffuser at the injection zone,

)1 the axial length of the curved diffuser, and

A s L,., wherein L is the maximum length of the tuyere as measured fromthe injection zone to the nose of the tuyere which for said givenfurnace, tuyere, and blast means conducts the fuel atomized in saidblast to the nose of said tuyere before ignition.

16. Method according to claim 3 wherein the generatrix of the curveddiffuser profile is defined by the following equation:

Y=R {1 [(R/r) 4 -11 [l-x/M wherein R radius of rotation of thegeneratrix at the discharge end of the curved diffuser profile portion,of the flow path,

blast to the nose of said tuyere before ignition. 17. In a tuyereaccording to claim 9 wherein the said other end of said curved diffuserterminates at said outlet nose without any linear extension.

18. Method according to claim I wherein the curved profile terminates atsaid other end at said nose.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,809,524 t d May 7, 1974 Inventor) Paul Bruhlet et a1.

It is certified that error appears 1T1 the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Claim 4 (Column 7), line 56 "hose" should be -whose--.

Signed and sealed this 24th day of September 1974.

(SEAL) Attest: McCOY M. GIBSON JR. c. MARSHALL D ANN Attesting OfficerCommissioner of Patents USCOMM-DC 60376-P69 u.s GOVERNMENT PRINTINGOFFICE: Ins 0-366-33.

FORM PO-105O (10-69)

1. In a method for blowing shaft furnaces through at least one tuyerewith a blast capable of containing large quantities of atomized liquidfuel, the improvement for maintaining a substantially constant impulseat the outlet of the tuyere, whatever the amount of fuel injected, withrapid combustion of the mixture at the nose of the tuyere, saidimprovement comprising the steps of: atomizing the liquid fuel in astabilized blast in an injection zone where the speed of said blast isbetween mach 0.3 and mach 1; conducting the resulting fuel-blast mixtureto the nose of the tuyere within a time which is less than the ignitiontime of the said mixture; constantly and smoothly decreasing the speedof the mixture towards the nose of the tuyere to the desiredfurnace-blowing velocity by coNstantly increasing the cross-section ofthe flow path along a curved diffuser profile the interior surface ofwhich is always convex and which profile at one end connects to theinjection zone and at its other end extends towards the nose of thetuyere and the tangent of which profile at said other end connects tosaid nose, which profile avoids recirculation steams.
 2. Methodaccording to claim 1 wherein the tangent to said curved profile hasfinite dimensions such that the said flow path is extended at thedischarge end by a linear profile.
 3. Method according to claim 1wherein said flow path has an outer boundary whose shape is whollydefined by a generatrix the equation for the curve of which has a secondderivative which is always positive.
 4. A method according to claim 2wherein the curved diffuser profile portion of said flow path has anouter boundary hose shape is defined by a generatrix the equation forthe curve of which has a second derivative which is always positive. 5.Method according to claim 3 wherein the tangent to the end of any curvedgeneratrix forms an angle of at most 15* with the tangent to the originof said generatrix.
 6. Method according to claim 4 wherein thegeneratrix of the linear flow path boundary is tangent to the curvedgeneratrix with which it connects and forms an angle of at most 15* withthe tangent to the origin of said generatrix.
 7. Method according toclaim 5 wherein said angle is between 10* and 12*.
 8. Method accordingto claim 6 wherein said angle is between 10* and 12*.
 9. In a shaftfurnace tuyere having a discharge conduit of fixed shape with a narrowedinlet at a fuel injection zone and an enlarged outlet nose and means forinjecting and atomizing variable amounts of liquid fuel into the blastflowing through said zone and into said discharge conduit, animprovement in said discharge conduit comprising means for supplyingthrough said injection zone a substantially stabilized blast at a speedof between mach 0.3 and 1, a curved diffuser with a constantly andsmoothly increasing cross-section throughout its length from said inletand extending tangentially at its other end towards said dischargeoutlet nose to flow said blast within said conduit without anyrecirculation and to deliver it at said discharge outlet nose atconventional furnace blowing velocities, said curved diffuser having ashape the generatrix of which is a curve whose equation has a secondderivative which is always positive, and the axial distance from saidinlet to said outlet nose being no greater than Lc.
 10. In a tuyereaccording to claim 9, the improvement further comprising a tangentiallinear extension to said diffuser.
 11. In a tuyere according to claim 9,the improvement further comprising the tangent to the end of any curvedgeneratrix forming an angle of at most 15* with the tangent to theorigin of said generatrix.
 12. In a tuyere according to claim 10, theimprovement further comprising the generatrix of the linear part beingtangent to the curved generatrix with which it connects and forming anangle of at most 15* with the tangent to the origin of said curvedgeneratrix.
 13. In a tuyere according to claim 11 the improvementfurther comprising the angle being between 10* and 12*.
 14. In a tuyereaccording to claim 12, the improvement further comprising the anglebeing between 10* and 12*.
 15. A tuyere according to claim 11, whereinthe generatrix of the curved diffuser is defined by the followingequation: Y R ( 1 + ((R/r) 4 -1) (1 - x/ lambda ) ) 1/4 wherein R radiusof the discharge end of the curved diffuser, which R is large enough toreduce a blast of at least 0.3 mach in the injection zone to withinacceptable furnace blowing velocities at said nose, r radius of theinlet of the diffuser at the injection zone, lambda the axial length ofthe curved diffuser, and lambda < or = Lc, wherein Lc is the maximumlength of the tuyere as measured from the injection zone to the nose ofthe tuyere which for said given furnace, tuyere, and blast meansconducts the fuel atomized in said blast to the nose of said tuyerebefore ignition.
 16. Method according to claim 3 wherein the generatrixof the curved diffuser profile is defined by the following equation: Y R( 1 + ((R/r) 4 -1) (1- x/ lambda ) ) 1/4 wherein R radius of rotation ofthe generatrix at the discharge end of the curved diffuser profileportion, of the flow path, r radius of rotation of the generatrix at theinjection zone end of the curved diffuser profile portion of the flowpath, lambda axial length of the curved diffuser, and lambda < or = Lc,wherein Lc is the maximum length of the tuyere as measured from theinjection zone to the nose of the tuyere which for said given furnace,tuyere, and blast conducts the fuel atomized in said blast to the noseof said tuyere before ignition.
 17. In a tuyere according to claim 9wherein the said other end of said curved diffuser terminates at saidoutlet nose without any linear extension.
 18. Method according to claim1 wherein the curved profile terminates at said other end at said nose.